1. Field of the Invention
This invention relates to the liquefaction of coal or similar liquefiable carbonaceous solids and is particularly concerned with hydrogen-donor solvent liquefaction processes in which liquids produced by the treatment of feed coal or similar solids with molecular hydrogen and a hydrogen-donor solvent are subsequently hydrogenated to produce recycle solvent and additional liquid products.
2. Description of the Prior Art
Coal liquefaction processes in which the feed coal is first contacted with molecular hydrogen and a hydrogen-donor solvent in a liquefaction zone at elevated temperature and pressure and a portion of the liquid product is then catalytically hydrogenated in a solvent hydrogenation zone to generate solvent for recycle to the liquefaction step and produce additional liquid products show promise as a means for the production of liquid hydrocarbons from coal. In such processes, hydrogenation of the liquid in the solvent boiling range is generally carried out at a pressure similar to or somewhat lower than that employed in the liquefaction zone and at a somewhat lower temperature. To supply the heat required to raise the solvent boiling range liquid to the hydrogenation temperature, it has been proposed that all of the vaporous product taken overhead from the liquefaction zone be passed directly to the solvent hydrogenation zone without cooling and that the quantity of coal liquids and recycle hydrogen which is mixed with the vaporous product and fed to the hydrogenation zone be adjusted so that the combined feed stream is maintained at the required hydrogenation temperature. This eliminates the need for a furnace to preheat the feed stream. Because the hydrogenation reaction is exothermic, additional cold feed is introduced into the hydrogenation zone downstream of the initial inlet point to quench the reaction and at the same time heat this additional feed to the necessary hydrogenation temperature.
The process described above has advantages over earlier processes from the standpoint of conserving thermal energy but poses certain operational problems which tend at least in part to offset the heat conservation advantage. The use of the liquefaction vapors to provide all of the heat needed to raise the initial increment of the liquid feed to the hydrogenation temperature and thus eliminate the need for a furnace limits the ratio in which the liquid and vapor can be introduced into the initial stage of the hydrogenation zone and imposes restrictions with respect to the hydrogen partial pressure in the initial stage. The cold feed introduced downstream of the initial stage has a shorter residence time within the hydrogenation zone than the feed introduced initially and hence uniform hydrogenation to achieve maximum solvent and product yields may be difficult to obtain. Overhydrogenation may sometimes occur. In addition, the introduction of relatively cold feed into the reaction zone at one or more points downstream of the initial inlet makes effective contacting of the feed and hydrogen more difficult to achieve, may promote product degradation and the production of excessive quantities of gas and low molecular weight hydrocarbons, and makes the overall reaction difficult to control. As a result of these and related disadvantages, the overall efficiency of such a process may leave much to be desired.